1. Field of the Invention
This invention relates to the inspection of meat products by x-ray means. More particularly, it relates to the fast determination of small bone fragments and other dense objects in conveyed meat products.
2. Background Information
Screening for bone and cartilage fragments in deboned poultry has become a major concern for the poultry industry. Manual palpation techniques are less than accurate, and pose a risk of spreading microbial contamination. Bones left in meat, in turn, result in product liability actions and shipment returns for rework with staggering cost implications [1], [2].
In recent years, x-ray systems have begun to emerge in bone screening applications. These systems look for differences in density from the raw poultry meat and bone or other foreign matter. Unfortunately, at the x-ray energies these systems typically employ, the density differences between bone and cartilage fragments and the raw meat itself are very small. In addition, the natural density variance in raw meat (created by water and fat) and meat thickness differences make it extremely difficult for simple transmission X-ray methods to accurately discriminate bone and cartilage.
The fraction of an x-ray beam that will penetrate an object is governed by the expression   I  =            I      0        ⁢          exp      ⁡              (                  -                                    ∫              l                        ⁢                                          μ                ⁡                                  (                                      E                    ,                    x                                    )                                            ⁢                              ⅆ                x                                                    )            
where I0 is the intensity of the incident x-ray beam, I is the intensity of the x-ray beam penetrating the object, xcexc(E,x) is the position and energy dependent x-ray attenuation coefficient, and 1 is the x-ray path length. The ratio I/I0 therefore depends upon the material properties of the object, the energy of the x-ray beam and the thickness of the object. Traditional x-ray screening systems measure only I, and therefore cannot consider the energy dependence of the attenuation coefficient or compensate for variations in the path length. Variations in the sample thickness can also prevent detection of foreign matter. If these thickness variations are known, however, mathematical compensation may be introduced.
Unlike traditional x-ray screening systems, which only measure I in Equation 1, the proposed system measures both I and I0 at several energies, and independently determines the x-ray path length. By more thoroughly measuring the parameters of Equation 1, the methods described below may be used to achieve greater sensitivity to bone, cartilage and other foreign fragments, and hence faster determinations of their presence in the poultry products.
A recent patent features dual x-ray scanners and simultaneous scanning of the interior and exterior portions of an animal carcass (U.S. Pat. No. Re. 36,664; Issued Apr. 18, 2000). Designed for segmenting animal carcasses, it is not adapted for making quick determinations of small bone fragments.
Another recent patent, also from the meat cutting field, employs flashing light sources or a single x-ray source to obtain rib thickness data of large animals (U.S. Pat. No. 5,902,177; Issued May 11, 1999). This patent also utilizes a laser ranging method for obtaining surface profile data.
While the above methods are suitable for segmenting large animal carcasses, they are not adapted for the quick determination of small bone fragments and other similarly dense objects in, for example, chicken breasts of non-uniform thickness traveling on a moving conveyor system.
In the medical imaging field, there have been several examples of the use of x-rays at two different energies to precisely distinguish bone and cartilage from muscle and fat. One patent features a switched-mode dual energy x-ray system where the different x-ray energies are obtained by periodic switching of the x-ray source voltage (U.S. Pat. No. 5,841,833, Issued Nov. 24, 1998).
1) xe2x80x9cThe Development of an On-Line X-Ray System that Accurately Detects Bone and Cartilage Fragments in Poultry Meatxe2x80x9d, U.S. Poultry and Egg Association, 1530 Cooledge Road, Tucker, Ga. 30084-7303.
2) xe2x80x9cNuclear Magnetic Resonance for Poultry Meat Fat Analysis and Bone Chip Detectionxe2x80x9d, U.S. Poultry and Egg Association, 1530 Cooledge Road, Tucker, Ga. 30084-7303.
3) R. E. Alvarez and A. Macovski, xe2x80x9cEnergy-Selective Reconstructions in X-ray Computerized Tomographyxe2x80x9d, Phys. Med. Biol. 21, 733-744 (1976).
4) S. S. Gleason, J. A. Mullens, M. J. Paulus, xe2x80x9cNext-Generation Reconstruction Algorithms for Computed Tomography: Seed Money Final Reportxe2x80x9d, ORNL Seed Money Final Report, Account # 3210-001Y (December, 1999).
At least two linear x-ray detectors are positioned below a conveyor belt in a poultry processing plant. At least two x-ray sources, each producing a different energy x-ray, are positioned above the conveyor belt for radiating the poultry. The x-rays that pass through the poultry, i.e., those that are not absorbed, are measured by the detectors. Laser profilometry is used to measure the chicken thickness as the x-ray data is acquired. The thickness data is used to compensate the x-ray profile measurement so that the x-ray attenuation will be accurately determined. The detector readout is processed in real time to detect the presence of highly attenuating fragments in the poultry such as bone, metal, or cartilage. A multiple-energy x-ray system has greater sensitivity to such foreign particles than a single energy system. It is believed that sub-millimeter resolution can be achieved.